As a fixing apparatus used in an image forming apparatus such as an electro-photographic copier or printer, there is a fixing apparatus for inserting a sheet of paper through a nipping section formed between a pair of rollers composed of a heat roller and a pressure roller or between similar belts and heating, pressurizing, and fixing a toner image. Such a heating type fixing apparatus, to maintain the heat roller at a fixed fixable temperature, detects the surface temperature of the heat roller by a temperature sensor and controls so as to turn a heating source ON or OFF according to detection results.
As a temperature sensor, in recent years, a temperature sensor of a non-contact type for detecting the temperature in no contact with the heat roller like an infrared temperature sensor has been used. The non-contact type temperature sensor does not damage the surface of the heat roller and the life span of the heat roller can be lengthened.
Such a non-contact type temperature sensor, to obtain high detection accuracy, must be positioned highly accurately to the heat roller and for example, in Japanese Patent Application Publication No. 2004-13024, a heat fixing apparatus in which a positioning pin of a temperature sensor is inserted into a positioning concavity formed in a fixing casing of a heat roller and the heat roller and temperature sensor are arranged at a fixed interval opposite to each other is disclosed.
Further, for example, in Japanese Patent Application 2002-294963, a fixing apparatus in which a temperature fuse as a temperature detection element is attached to a separation member arranged in the neighborhood of a heat roller and the temperature fuse is positioned to the heat roller is disclosed.
However, in these conventional temperature sensors, when the heat roller is exchanged or the temperature sensor is removed at the time of maintenance and is installed again, the interval between the heat roller and the temperature sensor is shifted due to the part accuracy and there is a fear of a reduction in the detection accuracy due to the temperature sensor. Therefore, in an apparatus in which a plurality of heating sources are dispersed and arranged in the axial direction and the heat rollers in the areas opposite to the respective heating sources are heated, there is a fear that the surface temperatures of the heat rollers in the axial direction become non-uniform. Therefore, the un-uniformity of the surface temperatures of the heat rollers appears in a fixed image and temperature ripple marks different in gloss are seen on the same image and the image quality is deteriorated.
On the other hand, in recent years, as a fixing apparatus of an induction heating method using an induction heating coil as a heating source, a fixing apparatus for installing a thinned metallic conductive layer having a small heat capacity on the surface of a heat roller to realize fast heating of the metallic conductive layer and realizing more energy conservation has been developed. In such a heat roller having a thinned metallic conductive layer with a small heat capacity, the temperature is changed greatly, so that when the detection accuracy of the temperature sensor is reduced, there is a fear that the precise surface temperature control of the heat roller may not be executed. Therefore, it is desired to improve the detection accuracy of the temperature sensor.
And, in a fixing apparatus for executing heating fixing, development of a fixing apparatus of an image forming apparatus in which a temperature sensor of a non-contact type for detecting the surface temperature of a heat roller is always positioned highly accurately to the heat roller, and the detection accuracy of the temperature sensor is improved, and the heat roller is accurately controlled in temperature, and the fixing property is improved, and a high image quality is obtained is desired.